Calcium carbonate precipitation inhibitor

The most complete study of the inhibition of calcium carbonate precipitation by organic matter was carried out by Berner et al. (1978), where primary concern was the lack of carbonate precipitation from supersaturated seawater. Both synthetic organic compounds and organic-rich pore waters from Long Island Sound were used to measure the inhibition of aragonite precipitation. Natural marine humic substances and certain aromatic acids were found to be the strongest inhibitors. The rate of precipitation in pore waters was also found to be strongly inhibited. 

Cathodic inhibitors reduce corrosion by rapidly polarizing the cathode. Examples of cathodic inhibitors are filming amines, magnesium salts, and calcium salts. In waters containing carbon dioxide, calcium carbonate precipitates (as discussed above) to form a protective film. 

Precipita.tingInhibitors. As discussed earlier, the localized pH at the cathode of the corrosion cell is elevated due to the generation of hydroxide ions. Precipitating inhibitors form complexes that are insoluble at this high pH (1—2 pH units above bulk water), but whose deposition can be controlled at the bulk water pH (typically 7—9 pH). A good example is zinc, which can precipitate as hydroxide, carbonate, or phosphate. Calcium carbonate and calcium orthophosphate are also precipitating inhibitors. Orthophosphate thus exhibits a dual mechanism, acting as both an anodic passivator and a cathodic precipitator. 

Ghelants and Precipitation Inhibitors vs Dispersants. Dispersants can inhibit crystal growth, but chelants, such as ethylenediaminetetraacetic acid [60-00-4] (EDTA), and pure precipitation inhibitors such as nitrilotris(methylene)tris-phosphonic acid [6419-19-8], commonly known as amino trismethylene phosphonic acid (ATMP), can be more effective under certain circumstances. Chelants can prevent scale by forming stoichiometric ring stmctures with polyvalent cations (such as calcium) to prevent interaction with anions (such as carbonate). Chelants interact... 

Moreover, although calcium carbonate scales can be successfully removed at production wells by acidizing or by using inhibitors, no long-term treatment exists to control silicate-containing precipitation. This is probably one reason that sodium orthosilicate is not frequently used in chemical flooding. Because of the plugging function, sodium silicate is mixed with calcium chloride alternately to improve sweep efficiency. 

The dramatic inhibitory effect of even trace amounts of PMA on the precipitation of calcium carbonate has long been recognised [249] and it has thus found considerable commercial application as an inhibitor of the formation of scale in hard water systems [251]. However, the mechanism by which this species inhibits the precipitation reaction remains unclear. Furthermore, to our knowledge, there have been no kinetic or morphological studies on the effect of polymaleic acid on the dissolution process. 

Cathodic inhibitors in near-neutral solutions interfere with the oxygen reduction reaction by restricting the diffusion of dissolved oxygen to the electrode surface. These substances usually form thick surface layers with poor electronic conductivity (the latter is an important prerequisite to avoid oxygen reduction on the film surface). Examples are phosphates, polyphosphates, silicates, borates, and inorganic inhibitors, such as Zn +, which precipitate as Zn hydroxide, and Ca ", which forms calcium carbonate films in the presence of CO3 [3]. These inhibitors... 

Cathodic precipitates increase cathodic site passivity with precipitation of insoluble compounds. Frequendy used cathodic precipitation inhibitors are CaCOs, MgCOs, or zinc sulfates that precipitate as Zn(OH)2. The efficiency of these inhibitors is only controlled by pH adjustment. Calcium carbonate (limestone) dissolves in water as calcium bicarbonate Ca(HC03)2- Carefid pH control forms smooth and hard calcium carbonate barrier films. Once the precipitate is formed, pH must be carefully controlled to avoid film dissolution at lower pH values ... 

NACE Test Methods for Determining Water Quality for Subsurface Injection Using Membrane Filters (TM0173) describes methods for evaluating water quality of injection water, NACE Test Methods for Laboratory Screening Tests to Determine the Ability of Scale Inhibitors to Prevent the Precipitation of Calcium Sulfate and Calcium Carbonate from Solution (for Oil and Gas Production Systems) (TM0374) is used to rank order scale inhibitors at 158°F (70°C). 

Inhibitor of the growth and agglomeration of the CaCOs formed when calcium is precipitated by the sodium carbonate often present in detergents. 

It can be seen that the long copolymer is able to act as crystal inhibitor for calcium carbonate only at very low temperatures (around 20°C). As soon as the wash temperature is raised to 40°C, there is no longer a crystal inhibition action, and at higher temperatures, the amount of precipitate even increases respect to the case of no polymer at all The polymer itself precipitates as bridging flocculant or, partially, as insoluble calcium salt. 

An important part of the water hardness is now eliminated by the sodium carbonate which, by becoming calcium carbonate, starts precipitating. The polymer is then called to perform its classical function of crystal inhibitor and antiredeposition agent, a function which still increases with the polymer level because we have a large excess of carbonate ions compared with the calcium counterions 800 ppm of carbonate ions are brought to the wash bath by the detergent itself, on top of the already present natural water hardness. 

Cathodic inhibitors Inhibitors which decrease the corrosion rate by suppression of cathodic reduction reaction, such as oxygen and hydrogen reduction. Cathodic precipitates Inhibitors, like calcium carbonate and magnesium carbonate, which precipitate on cathodic areas. 

The most widely used cathodic precipitation-type inhibitors are the carbonates of calcium and magnesium because they occur in natural waters and their use as an inhibitor usually requires only an adjustment of pH. Zinc sulfate (ZnSO ) precipitates as zinc hydroxide ZnjOHjj on cathodic areas and is considered an inhibitor of this type. Phosphates and silicates are not distinctively cathodic or anodic inhibitors, but appear to be a combination of both types, so they will be considered later in the section on precipitation inhibitors. 

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